One method for imposing an electrical signal onto an optical carrier is to use an external modulator. Examples of external modulators presently in use include Mach-Zehnder (MZ) and electro-absorption (EA) types. In most cases, pyroelectric, photorefractive and photoconductive effects in the device material of a modulator (often lithium niobate, or a semiconductor like GaAs, or an electro-optic polymer) can cause its output power-vs-input voltage characteristic (i.e., its transfer function) to change or “drift,” over time or temperature. The result of drift is that a specific DC bias voltage (or even 0 Volts) may, for example, yield a minimum point on the transfer function curve at one time, but yield a different point on the curve at a later time and/or at a different temperature. This situation is illustrated in FIG. 1 for a digital application of a Mach-Zehnder type of external modulator in which the same digital electrical input signal produces two very different optical output signals, depending on the location of the operating point on the modulator transfer function.
There have been many attempts either to eliminate the drift problem through better modulator design or to circumvent it through the use of auxiliary circuits that maintain a fixed bias point. Since it appears that there has not been a commercially viable solution using the former approach, the use of bias controllers that implement the latter approach has become the standard method for dealing with the modulator drift problem.
Many prior art methods for maintaining a specific bias point have assumed a Mach-Zehnder modulator. The modulation transfer function of an MZ modulator is such that the intensity of light passing through it varies in a sinusoidal fashion to the applied voltage, as shown in FIG. 2. The magnitude of the control voltage change required to transition from a maximum (or minimum) to a minimum (or maximum) is generally stable; and is known as the switching voltage (often designated as Vπ). It can be measured for each individual modulator and will have a fixed value. The four most commonly utilized DC operating bias points for the MZ modulator are known (by their relationship to the output Intensity) as the maximum (‘Max’), minimum (‘Min’), or one of two half-way points (‘Quad +’ and ‘Quad −’, depending on whether the intensity is increasing or decreasing with increasing applied voltage).
FIG. 3 shows a representative EA modulator transfer function. While the output optical intensity from interferometric modulators like the MZ type vary periodically with voltage, the EA modulator transitions only once between “on” (minimum optical absorption) and “off” (maximum absorption) when the voltage is changed by a magnitude Vs. Therefore there is no “Quad +” bias point for an EA modulator.
The most common approach to measuring the operating bias point is to apply a small, low-frequency signal to the modulator. Such a signal is referred to as a dither signal, because it dithers or perturbs the operating bias point. By measuring the optical modulation that results from applying these perturbations—and knowing the functional form of the modulator transfer function—it is possible to determine the present operating bias point. For example, at the principal points on a modulator's transfer function curve—i.e., the maximum, quadrature, and null points—the signal amplitude at either the fundamental frequency (or frequencies) of a dither signal or second-order distortion products of this signal is minimized.
FIG. 4 shows a prior art dither-based modulator bias controller. Along with a variable DC bias voltage, the controller applies a periodic, low-frequency, small-amplitude dither signal to the MZ modulator electrodes. A small fraction of the optical output of the modulator is diverted to a detector in the bias control circuit, and the output of this detector is routed through a filter. The filter rejects all frequencies other than the frequency of the dither signal required for controlling the desired bias point. The feedback circuitry continually adjusts the DC bias to the modulator to minimize the level of the signal that is passed through the filter.
These prior art methods for controlling the bias point on an optical modulator are believed to fail to effectively maintain an optimum bias point when a large modulation signal is applied to the modulator, as is the case in digital telecommunication applications. Unlike these prior art methods, embodiments according to the invention described in this application can obtain and maintain the correct bias point on a modulator's transfer function curve when the modulation signal has a modulation depth on the order of 100% (e.g., when the peak-to-peak voltage of a digital modulation signal is on the order of the modulator's on-to-off switching voltage).